Non-cryogenic bulk oxygen separation systems, for example, organic polymer membrane systems, have been used to separate selected gases from air and other gas mixtures. Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure.
Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500.degree. C., generally in the 600.degree. C. to 900.degree. C. range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art. Since the selectivity of these materials for oxygen is infinite and oxygen fluxes several orders of magnitude higher than for polymeric membranes can be obtained, attractive opportunities are created for the production of oxygen as well as for oxygen-requiring oxidation processes, especially with applications that involve elevated temperatures, for example, coal gasification.
Coal gasification has the potential of utilizing coal reserves for the production of synthetic fuels. Several commercial processes have been developed to produce low-BTU, medium-BTU, and high-BTU gases from coal. Many different types of coal gasification processes are known in the art, for example, fixed bed, fluidized bed, entrained bed, and molten bath bed. These processes can be catalytic or noncatalytic, and each is carried out under widely different conditions. Some of the major gasification processes are described in Chapter 6 of Coal Liquefaction and Gasification Technologies, E. C. Mangold et al. (1982). The aspect common to most of these coal gasification processes is the use of steam and oxygen and/or air to carry out a partial and/or complete oxidation of the coal. The steam reforming of coal at high temperatures produces a fuel gas stream containing mainly carbon monoxide and hydrogen gas. Additional steps, however, such as water gas shift reaction and methanation, may be carried out to adjust the composition of the product gas stream. Due to the complex chemistry of coal, other reactions may take place and higher hydrocarbons and tar may also be formed during the coal gasification process.
When air is used as the oxidant, coal gasification generally produces low-BTU gas since the nitrogen in the air stream acts as a diluent. Therefore, use of oxygen or an oxygen-enriched gas stream is generally necessary to form medium-BTU to high-BTU gas stream. In this case, the cost of the required oxygen gas is a significant fraction of the overall coal gasification cost. Thus, the possible integration of coal gasification with a high temperature oxygen production process could be advantageous.
A class of processes that integrate thermal power generation with coal gasification may be referred to as integrated gasification power cycles (IGPC). An integrated gasification combined cycle (IGCC) is a specific embodiment of such a scheme and IGCC is well-known in the art.
In an IGCC (also known as Combined Cycle Coal Gasification, or CCCG) plant, coal is gasified to a fuel gas stream which is supplied to gas turbines employed in a combined cycle power generation system. Thermal integration between coal gasification and power production processes results in improved overall efficiency of the plant.
In gas turbine power cycles, the working fluid is generally air or the products of combustion of fuel and air. Heat is generated within the system by combusting fuel in a compressed air stream and the products of combustion are expanded through a gas turbine to produce power. The metallurgical temperature limit on the turbine blades necessitates a gas turbine operation with a very high oxygen/fuel ratio: nitrogen in the feed air and the excess oxygen act as diluents and lower the temperature of the combustion products. As a result, the exhaust gas stream from the gas turbine power cycle contains excess oxygen which could combust additional fuel. The hot exhaust gas stream could be used to preheat the compressed feed air stream or may also be employed to generate steam that can be used in a vapor power cycle, for example, (a Rankine cycle or its modifications such as reheat and regenerative cycles, dual pressure cycle, and cogeneration cycles).
In a gas power cycle, it is also possible to recover some or all of the excess oxygen in the oxidant gas stream (generally air) either before or after the gas turbine. This oxygen recovery is done at the expense of compressing additional feed air to replace the oxygen removed and the capital costs associated with the oxygen removal system. The oxygen recovery process produces oxygen at a very low incremental power cost, which is attractive compared to other methods of oxygen production. The gases from which oxygen needs to be separated are made available at relatively high temperature (greater than 400.degree. C.), whereas current commercial oxygen production processes typically operate at temperatures below 100.degree. C. Because of this temperature limitation, current commercial oxygen production processes do not gain significantly by integration with a coal gasification process. It appears that novel gas separation processes employing oxygen-ion conductor membranes have the promise of highly synergistic integration which can dramatically lower the cost of oxygen used in coal gasification processes.
Most oxygen generating systems utilize cryogenic separation methods (generally for large scale, high purity applications) or use polymeric membrane or adsorptive separation techniques (generally for small to medium scale, 90-95% purity applications). Membrane systems are typically very power intensive, and are suitable only for the production of small quantities of oxygen-enriched air (for example, 50% oxygen). Some of these processes recover a part of the power utilized in producing the product, however they do not produce any net power.
As mentioned above, traditional oxygen separation processes operate at low temperatures (less than 100.degree. C.), and do not benefit significantly from integration with high temperature processes that utilize oxygen. The elevated temperatures of operation make the ion transport process intrinsically well suited for integration with high temperature processes such as coal gasification and combined cycle power generation, as described in the following references.
A JPL publication D-7790 (1990) has disclosed integration of a high temperature oxygen production process using a zirconia-based oxygen-ion conductor within a CCCG configuration. In this process, oxygen extracted from air by the oxygen-ion conductor is used for coal gasification. Oxygen-depleted air is combusted with the fuel gas produced in the coal gasifier, and expanded in a gas turbine to generate power. The gas turbine exhaust is used to produce steam, which is expanded in a steam turbine to generate additional power.
U.S. Pat. No. 5,306,411 (Mazanec et al.) discusses mixing an inert diluent such as steam with a light hydrocarbon feed gas to produce synthesis gas or unsaturated hydrocarbons as a permeate effluent from the anode side of an ion transport membrane. An oxygen-containing gas is passed through a retentate or cathode side of the membrane; the resulting oxygen-depleted gas withdrawn from the retentate zone apparently is discarded. Mazanec at al. further disclose in U.S. Pat. No. 5,160,713 that steam can be generated in the permeate zone by reaction between hydrogen, introduced as an oxygen-consuming substrate, and oxygen transported through the membrane.
Kang et al., U.S. Pat. No. 5,565,017, entitled High Temperature Oxygen Production with Steam and Power Generation, relates to a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated and steam is added. Water is added to the retentate gas stream from the ion transport module prior to the gas turbine to increase the mass flow in the turbine. This permits the ion transport module and the gas turbine to each operate at its optimum temperature.
In Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, feed air is compressed and heated in a first heating step (using heat exchanger and combustor) before passing through an oxygen separator employing a mixed conducting oxide. The retentate gas stream from the ion transport module is heated in a second heating step before expanding it through a gas turbine to recover power. The hot exhaust gases from the gas turbine are used to produce steam that is expanded through a steam turbine to generate additional power. In these processes, the operating temperatures of the ion transport module and the gas turbine are independently maintained by controlling the rate of heat addition in the first and second heating steps.
U.S. Pat. No. 5,562,754 (Kang et al.) entitled "Production of Oxygen by Ion Transport Membranes with Steam Utilization" discloses ion transport-based oxygen production integrated with gas turbine power production, in which the permeate side of the ion transport module is swept with an inert sweep gas such as steam to enhance the oxygen flux across the ion transport membrane. As a result a stream containing a mixture of steam and oxygen is produced on the permeate side and can be withdrawn as a product. Kang suggests that this stream can be at a pressure from 2 to 300 psi and could be used to feed a gasifier requiring both steam and oxygen. A closer inspection of Kang's concept reveals that it does not provide a practical solution for generating a steam-oxygen mixture of the required composition and at the required pressure for many of the more practical coal gasification processes. The reasons for this are several: i) Gasification reactors typically operate at pressures well exceeding 300 psia and require steam and oxygen at pressures exceeding 350 psi; ii) It is energy intensive, expensive, potentially unsafe and therefore impractical to compress a steam-oxygen mixture in a compressor; iii) The steam-to-oxygen molar ratio required by the gasification process is typically close to 1, although there are exceptions where it is higher. Using an improbable maximum example having a separator retentate pressure of 300 psia and inlet composition of 20% oxygen, the retentate inlet oxygen partial pressure would be 60 psia which would also be the absolute limit for the oxygen partial pressure at the permeate side pinching end. Under those conditions and at a permeate steam pressure of 350 psia the limiting molar ratio of steam to oxygen would be 4.8, exceeding significantly the typical required value of approximately 1.